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Home , Space geodesy

Journal of Geodynamics 72 (2013) 1–10

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Journal of Geodynamics

j ournal homepage: http://www.elsevier.com/locate/jog

Observing and understanding the Earth system variations from space geodesy

a,∗ b c

Shuanggen Jin , Tonie van Dam , Shimon Wdowinski

Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China

University of Luxembourg, Luxembourg L-1359, Luxembourg

University of Miami, Miami, FL 33149, USA

a r t i c l e i n f o a b s t r a c t

Article history: The interaction and coupling of the Earth system components that include the atmosphere, hydrosphere,

Received 4 July 2013

cryosphere, lithosphere, and other fluids in Earth’s interior, influence the Earth’s shape, gravity field and

Received in revised form 5 August 2013

its rotation (the three pillars of geodesy). The effects of global climate change, such as sea level rise,

Accepted 6 August 2013

glacier melting, and geoharzards, also affect these observables. However, observations and models of

Available online 16 August 2013

Earth’s system change have large uncertainties due to the lack of direct high temporal–spatial mea-

surements. Nowadays, space geodetic techniques, particularly GNSS, VLBI, SLR, DORIS, InSAR, satellite

Keywords:

gravimetry and altimetry provide a unique opportunity to monitor and, therefore, understand the pro-

Space geodesy

cesses and feedback mechanisms of the Earth system with high resolution and precision. In this paper,

Earth system

Interaction the status of current space geodetic techniques, some recent observations, and interpretations of those

Climate change observations in terms of the Earth system are presented. These results include the role of space geo-

detic techniques, atmospheric–ionospheric sounding, ocean monitoring, hydrologic sensing, cryosphere

mapping, crustal deformation and loading displacements, gravity ﬁeld, geocenter motion, Earth’s oblate-

ness variations, Earth rotation and atmospheric-solid earth coupling, etc. The remaining questions and

challenges regarding observing and understanding the Earth system are discussed.

1. Introduction they also have a low-spatial resolution, e.g., low temporal resolu-

tion radiosonde balloons that are launched twice per day (Jin and

The Earth system varies continuously due to interactions and Luo, 2009; Jin et al., 2009). Another example is ionospheric mon-

coupling between its main ﬂuid components that include the atmo- itoring, in which traditional instruments are very expensive and

sphere, the hydrosphere, the cryosphere, the lithosphere, and the provide only low spatial sampling. Furthermore, ionosondes can-

Earth’s interior. Mass redistributions driven by geodynamic pro- not measure the topside ionosphere and sometimes suffer from

cesses in the Earth system affect the Earth’s shape, gravity ﬁeld absorption during storms, whereas Incoherent Scatter Radar (ISR)

and rotation. However, the system and components are also being has geographical limitations, and even low-Earth-orbiting satellite

affected by global climate change. The Intergovernmental Panel on (altitudes of 400–800 km) observations cannot provide information

Climate Change (IPCC) of United Nations reported that the global on the whole ionosphere (Jin et al., 2006).

climate has been warming since the middle of the 20th century Oceans have been observed by drifting buoys and tide gauges

due to increases in atmospheric greenhouse gas concentrations (TG) for almost two centuries (Barnett, 1984). Mean sea level rise

(Pachauri and Reisinger, 2007). To conﬁrm and assess the future was observed using TG through the 20th century. Sea-level rise

effects of global warming, still requires additional observations. was thought to be driven by a steric component due to the thermal

However, traditional meteorological sensors suffer from a number expansion of the oceans and a non-steric component due to fresh

of limitations. In addition to being highly labor-intensive to deploy water input from the melting of continental ice sheets and glaciers

(Holgate and Woodworth, 2004; Church and White, 2006; Feng

et al., 2013), that affect our living environments, marine ecosys-

tems, coasts and marshes. However, TG provides the relative sea

∗

Corresponding author at: Shanghai Astronomical Observatory, Chinese

level variations with respect to the land. Furthermore, TG data

Academy of Sciences, Shanghai 200030, China. Tel.: +86 21 34775292;

are point measurements and are provided at low spatial resolu-

fax: +86 21 64384618.

tions. We cannot quantify the steric contribution to the sea level

E-mail addresses: [email protected], [email protected] (S. Jin),

[email protected] (T. van Dam), [email protected] (S. Wdowinski). budget due to the lack of global ocean temperature and salinity

2 S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10

observations. In addition, detailed ocean circulation and the labor intensive. The advent of space geodesy has represented a

exchange between land and ocean water on a global scale are dif- revolution for the space age and Earth exploration, e.g., VLBI, SLR,

ﬁcult to measure using traditional instruments. DORIS, GNSS, InSAR/LiDAR, and satellite radar and laser altimetry

The solid-Earth’s surface and interior are changing constantly (see Fig. 1). These space geodetic techniques are capable of mea-

because of mantle convection, tectonics, and surface processes. suring and precisely monitoring small changes of the parameters

These activities cause displacements and deformations of the they observe with high spatial–temporal resolution. The different

Earth’s surface, landslides and subsidence, mud-rock ﬂow, and roles and uses of each space geodetic technique are summarized

other phenomena. In the past, plate motion was inferred from in Table 1. In the following discussion, the main space geodetic

marine magnetic anomalies located on both sides of the mid-ocean techniques are brieﬂy introduced.

ridges and from the azimuth of transform-faults (at time scales

of millions of years), as well as from historic earthquake source 2.1. GNSS

parameters. Another issue is that it is difﬁcult to monitor present-

day intraplate crustal deformations in sufﬁcient details (Jin and Zhu, Global Navigation Satellite System (GNSS) is a recent term used

2002). For example, current deformation in East Asia is distributed to describe the various satellite navigation systems, such as GPS,

over a broad area extending from Tibet in the south to the Kuril- GLONASS, Beidou and Galileo. The Global Positioning System (GPS)

Japan trench in the east with some micro-plates, such as South with its unprecedented precision, has provided great contributions

China and possibly the Amurian plate, embedded in the deform- to navigation, positioning, timing and scientiﬁc questions related

ing zone (Jin et al., 2007). Because of the sparse seismicity in the to precise positioning on Earth’s surface, since it became fully

region and the lack of clear geographical boundaries in the broad operational in 1994 (Jin et al., 2011a). Today, a number of Earth

plate deformation zones, it has been difﬁcult to describe accurately system science questions have been successfully investigated,

the tectonic kinematics and plate boundaries acting here. Earth- including the establishment of a high precision International Ter-

quakes and volcanoes frequently occur worldwide, with notable restrial Reference Frame (ITRF), Earth rotation, geocenter motion,

cases including: the Mw = 9.1 Sumatra Earthquake in 2004; the time-variability of the gravity ﬁeld, orbit determination as well as

Mw = 8.1 Wenchuan Earthquake in 2008; the Mw = 8.0 Chile Earth- remote sensing of the atmosphere, hydrology and oceans. With

quake in 2010; and the Mw = 9.0 Tohoku Earthquake in 2011. These the development of the next generation of multi-frequency and

earthquakes took thousands of lives and generated huge tsunamis. multi-system GNSS constellations, including the U.S.’s modernized

Seismometers around the globe can estimate the nature of earth- GPS-IIF and planned GPS-III, Russia’s restored GLONASS, the coming

quakes, but the details of rapid rupture are usually obscured by the European Union’s GALILEO system, and China’s Beidou/COMPASS

lack of near-ﬁeld near-real-time observations. In addition, observ- system, as well as a number of regional systems including Japan’s

ing and modeling Earth’s interior activities are still challenging due Quasi-Zenith Satellite System (QZSS) and India’s Regional Naviga-

to the complex mass transport and other physical processes acting tion Satellite Systems (IRNSS), more applications and opportunities

within the Earth system (Jin et al., 2010a; Jin and Feng, 2013). will be realized to explore the Earth system using ground and space

Therefore, it is hard to observe and model Earth system and envi- borne GNSS.

ronment changes. Today, Earth observations from space provide a

unique opportunity to monitor variations in the various compo- 2.2. VLBI

nents of the Earth system. Observations include data from the dense

distribution of ground global positioning system (GPS) stations; Very-long-baseline interferometry, VLBI, plays a key role in

high resolution space-borne GPS radio occultation; long-term Very space geodesy by receiving natural quasars signals at two or

Long Baseline Interferometry (VLBI); Satellite Laser Ranging (SLR); more Earth-based radio telescopes. VLBI is particularly impor-

Doppler Orbitography and Radiopositioning Integrated by Satellite tant in establishing the International Celestial Reference Frame

(DORIS); Interferometric Synthetic Aperture Radar (InSAR); Light (ICRF). With the global VLBI tracking network, VLBI can be used

Detection and Ranging (LiDAR); satellite radar and laser altime- to monitor plate motion, crustal deformation, polar motion and

try (Abshire et al., 2005); and satellite gravimetry (particularly length-of-day as well as being used for deep space exploration.

CHAMP/GRACE/GOCE) (e.g., Tapley et al., 2004). These observa- The “VLBI2010: Current and Future Requirements for Geodetic

tions allow us to measure and monitor small displacements of the VLBI Systems” provided a path to a next-generation system with

Earth’s surface and mass transport with high precision and high unprecedented new capabilities and accuracies, including 1 mm

spatial–temporal resolutions. These data provide a unique oppor- position and 0.1 mm/year velocity measurement accuracy, con-

tunity to investigate mass transport associated with geodynamics, tinuous observations for station positions and Earth orientation

natural hazards, and climate change and to better understand the parameters. Additional applications will be developed in the near

processes themselves and their interactions within the Earth sys- future (Titov, 2010). For example, the Chinese VLBI Network (CVN)

tem. In this paper, the different space geodetic techniques and Earth has been upgraded (e.g., two VLBI radio telescopes in Shanghai

system interactions are introduced. Recent observations and mod- and Urumqi, and several proposed new VLBI2010-type system) and

eling results from space geodesy that allow us to infer changes in new applications such as successfully tracking the China’s Chang’E-

the atmosphere, oceans, hydrosphere, cryosphere, lithosphere, and 1/2 lunar exploration probes (Wei et al., 2013).

the Earth’s deep interior are presented.

2.3. SLR/LLR

2. Space geodetic techniques and observations Satellite Laser Ranging (SLR) systems precisely determine the

distance of the satellites above the Earth’s geocenter by measuring

The word “geodesy” etymologically comes from the Greek the time to send and receive short laser pulses. These data have

“geôdaisia”: “dividing the Earth”. Today, geodesy is deﬁned as been widely used in precise orbit determination (POD) of artiﬁcial

measuring the Earth’s size, shape, orientation, gravitational ﬁeld satellites and station motions. In addition, since the low-earth-orbit

and their variations with time using geodetic techniques, e.g., (LEO) laser ranging satellites are sensitive to the low-degree gravity

Arc measurements (historic), Triangulation, Trilateration, Travers- ﬁeld, SLR is considered the gold-standard in monitoring geocenter

ing, Leveling, Zenith or vertical angles measurement and ground and C20 (Cheng and Tapley, 2004; Jin et al., 2011b). Lunar Laser

gravimetry. However, these traditional geodetic techniques are Ranging (LLR) can measure the distance between the moon and

S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10 3

Fig. 1. Main space geodetic techniques.

Earth, which allows us to check certain general relativity principals potentially measure displacement of the Earth’s surface over time

and explore rotational properties of the moon. spans of days to years with centimeter level precision. Recently,

many varied applications of InSAR have been published, e.g., moni-

toring earthquakes, volcanoes, landslides, hydrological cycle, ocean

2.4. DORIS

environment, and glacier movement. In addition, tropospheric and

ionospheric delays can be extracted using the InSAR technique.

Doppler Orbitography and Radio positioning Integrated by

These data can be used in meteorology and space weather.

Satellite (DORIS) is a technique whereby satellites receive the

broadcasted wavelength signal transmitted by ground stations. As

the satellite moves, the received frequency is shifted. The satellite

2.6. Satellite altimetry

velocity and position are linked by the Keplerian equation of its

orbit, so the Doppler shift allowing us to determine the station to

Satellite altimetric observations have promoted and advanced

satellite distance. DORIS can determine the orbit of the transmit-

oceanographic research since the ﬁrst mission Topex/Poseidon in

ting satellites, Earth rotation and station coordinates that can be

1993 as well as later GEOSAT Follow-On (GFO), ERS-2, Jason-1/2,

used in different geodetic applications (see Willis et al., 2010).

and Envisat. A number of results have been obtained with almost

covering the whole Earth at centimeter accuracy, e.g., the mean sea

surface height (MSSH), sea level change and ocean circulation as

2.5. InSAR

well as El Nino events monitoring (Feng et al., 2013). In addition,

laser altimeter also can monitor glacier melting. For example, the

Interferometric Synthetic Aperture Radar (InSAR) can be used

Geoscience Laser Altimeter System (GLAS) on bard ICESat can mon-

monitor surface deformation or determine digital elevation model

itor the ice sheet mass balances (Zwally et al., 2005; Abshire et al.,

(DEM) using differences between two or more synthetic aperture

2005).

radar (SAR) images (Massonnet and Feigl, 1998). The technique can

Table 1

Roles and uses of each space geodetic technique.

Parameters GNSS VLBI DORIS SLR LLR Altimetry InSAR/LiDAR Gravimetry √

ICRF √ √ √

√ √ √ √ √

ITRF ( ) ( ) ( )

√ √ √ √ √ √

Polar motion √ √ √ √ ( )

Nutation ( ) √ ( )

UT1 √

√ √ √ √ √

Length of day ( )

√ √ √ √ √

Geocenter ( )

√ √ √ √ √ √ √

Gravity ﬁeld √ ( ) √ √

√ √ √ √ √

Plate motion √ ( ) ( )

√√

√ √ √

LEO orbits √

√ √ √ √ √

Navigation/orbit

√ √ √

Timing and clock ( )

√ √ √ √ √ Ionosphere √√√ √ √

Troposphere √

√ √ √ Hydrology √ √√ √ Oceans

4 S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10

Space Techniques Earth System GNS S Earth Atmosphere

VLBI Rotation Oceans

DORIS Geomet ry & Hydrosphere

SLR/LLR Deformati on Cryosphere

InSA R Crust Mantle LiDAR Core Altimetry …

Satellite Gravimetry Tides

… Load ings

Geocenter motions

Terrestrial Techniques Tectonic motions Leveling Gravity Volcanoe s

Tide Gauge Field Earthquakes

Gra vimetry Tsunamis

Reference

Tiangulation GIA

Frame … Landslides

….

Fig. 3. Space geodesy and earth system interaction.

affect the Earth’s rotation, geometry and deformation and gravity

ﬁeld as well as reference frame.

Fig. 2. Earth system components and interaction.

4. Observing and understanding Earth system

2.7. Satellite gravimetry

4.1. Atmospheric sounding

With the recent development of the low-earth orbit (LEO)

satellite gravimetry, the precision and temporal resolution of the Space techniques’ radio signals propagate through the Earth’s

Earth’s gravity ﬁeld model has been greatly improved. Satellite neutral atmosphere and ionosphere. This results in lengthening

gravimetry is a successful innovation and breakthrough in the the geometric path of the ray, usually referred to as the “tro-

ﬁeld of geodesy, following the Global Positioning System (GPS). pospheric delay and ionospheric delay.” These delays are one of

Unlike the traditional terrestrial gravity measurements, the most major error sources for space techniques. Today, the space tech-

advanced SST (Satellite-to-Satellite Tracking) and SGG (Satellite niques such as GPS, VLBI, InSAR, DORIS and altimetry are able to

Gravity Gradiometry) techniques are used to estimate the global measure such delays and corresponding precipitable water vapor

high-precision gravity ﬁeld and its variations (Knudsen et al., (PWV) and ionospheric total electron content (TEC) with a high

2011). Satellite-to-Satellite Tracking technique includes the so- resolution and precision. These products have been widely applied

called high–low satellite-to-satellite tracking (hl-SST) and low–low in meteorology, climatology, numerical weather models, atmo-

satellite-to-satellite tracking (ll-SST) (Wolff, 1969), which can pre- spheric science and space weather (e.g., Jin et al., 2006; Jin and

cisely determine the variation rate of the distance between two Park, 2007; Jin et al., 2008, 2009; Jin and Luo, 2009). For example,

satellites. Satellite gravimetry, particularly recent Gravity Recovery Fig. 4 shows the long-term vertical TEC variations from GPS at the

and Climate Experiment (GRACE), can monitor the mass transport

and redistribution in the Earth system, which has been widely

o o

V TEC at (Lat: -87.5 ; Lon: 120 )

applied in geodesy, oceanography, hydrology, ice mass change, geo-

dynamics and geophysics (Wahr et al., 1998; Tapley et al., 2004;

Famiglietti et al., 2011; Jin and Feng, 2013). 40

CU)

3. Space geodesy and earth system interaction TE 20 C (

The Earth system is very complex due to the interaction and

V 10

coupling of its components (Fig. 2). For example, melting conti-

nental glaciers will result in rising sea levels, tectonic activities and

reservoir accumulated water may lead to hazards, e.g., earthquakes, 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

tsunamis, landslides and mud-rock ﬂows and the atmospheric Time (year)

pressure loading will change the shape, geocenter, oblateness and

10.7 cm Solar Flux

rotation of the Earth, etc. 300

)

Today space geodetic techniques with high precision and -1

temporal–spatial resolution can measure and monitor such

-2

changes within the Earth system, including Earth rotation, 200

Earth’s oblateness, geocenter, geometry and deformation, grav-

-22

ity ﬁeld, atmosphere, oceans, hydrosphere, cryosphere, crust,

10 100 ocean/atmospheric/polar tides, ocean/atmospheric/hydrological

Flux (

loading, tectonic motions, volcanoes, earthquakes, tsunamis,

glacier istostatic adjustment (GIA) and landslides, etc. Fig. 3 shows

the role of space geodesy in observing and understanding the Earth 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

system. The reference frame is a benchmark for Earth rotation,

Time (year)

geometry and deformation and gravity ﬁeld measurements that can

◦ ◦

be precisely determined using space geodesy. Earth system changes Fig. 4. Vertical TEC (VTEC) time series at (120 E , −87.5 S ) and 10.7 cm Solar Flux.

S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10 5

◦ ◦

point (120 E , −87.5 S ), which are mainly affected by solar activity.

Additionally, the space-borne GPS radio occultation (RO) missions

can estimate atmospheric and ionospheric products, including the

pressure, temperature, water vapor, total electron content (TEC)

and electron density as well as their variation characteristics, e.g.,

the US/Argentina SAC-C, German CHAMP (CHAllenging Minisatel-

lite Payload), US/Germany GRACE (Gravity Recovery and Climate

Experiment), Taiwan/US FORMOSAT-3/COSMIC (FORMOsa SATel-

lite mission – 3/Constellation Observing System for Meteorology,

Ionosphere and Climate) satellites, the German TerraSAR-X satel-

lites and the European MetOp (Schmidt et al., 2010; Jin et al., 2011a).

4.2. Ocean monitoring

The oceans have been well monitored by satellite altimetry for

about two decades. These data provide information on the mean

sea surface height (MSSH), ocean circulation, tides and sea level

change. The sea level change includes the steric and non-steric sea

changes. While the steric sea level change, i.e., thermal expansion,

is well estimated by Argo observations, the non-steric component,

i.e., eustatic sea level change, related to mass changes driven by the

addition of water to the oceans from the melting of continental ice

sheets and fresh water in rivers and lakes (Feng et al., 2013) is more

difﬁcult to estimate. The difﬁculty is due to the highly uncertain

estimates of the Antarctic and Greenland mass and land reservoirs.

The satellite-based GRACE observations provide a unique oppor-

tunity to directly measure the global ocean and continental water

mass change. For example, Fig. 5 shows the sea level change in

cm/year from satellite altimetry, Argo and GRACE (2002–2011),

which generally quantify the total sea level change budget.

In addition, GNSS-Reﬂectometry (GNSS-R) is a new innova-

tive and promising approach in ocean remote sensing. In addition,

the technique poses many potential advantages (Jin et al., 2011a),

including primarily global coverage and long-term satellite mission

lifetime. In the last few years, several experiments were undertaken

and numerous advancements were realized. The ﬁrst GPS signals

reﬂected from the sea surface inside tropical cyclones were ana-

lyzed, and the wind speed results were obtained (Katzberg et al.,

2001). Currently, the GPS reﬂected signals from the ocean sur-

face can measure sea surface height with the achievable accuracy

(Martin-Neira et al., 2001; Katzberg and Dunion, 2009).

4.3. Hydrologic sensing

The power level of the GPS reﬂected signal from the land

contains information about the soil moisture, dielectric constant,

surface roughness, and possible vegetative cover of the reﬂect-

ing surface. Katzberg et al. (2006) obtained the soil reﬂectivity

and dielectric constant using a GPS reﬂectometer installed on an

HC130 aircraft during the Soil Moisture Experiment 2002 (SMEX02)

near Ames, Iowa, which were consistent with results found for

other microwave techniques operating at L-band. In addition,

the multi-path from ground GPS networks is possibly related to

Fig. 5. Sea level change in cm/year from satellite altimetry, Argo and GRACE

the near-surface soil moisture. Larson et al. (2008) found nearly

(2002–2011).

consistent ﬂuctuations in near-surface soil moisture from the

ground-based observations of GPS multi-path. They found GPS esti-

mates of soil moisture were comparable to estimates in the top 5 cm storage reserves with about 20.5 mm/year due to recent ﬂoods. In

of soil measured from conventional sensors. the La Plata region of South America, the terrestrial water storage

−

Satellite gravimetry, in particular GRACE, can determine the is falling at a rate of 9.8 mm/year due to recent droughts.

monthly terrestrial water storage change. Fig. 6 shows the trend of

terrestrial water storage change in the world based on GRACE data 4.4. Wetland mentoring by InSAR

(August 2002–February 2011), which reﬂect groundwater deple-

tion, drought and ﬂoods. For example, the terrestrial water storage Wetland InSAR is a unique application of the InSAR technique,

is falling by about −15.5 mm/year in the northwest region of India because unlike all other applications that detect displacements of

due to groundwater depletion (Rodell et al., 2009). The Amazon solid surfaces, wetland InSAR detects changes of aquatic surfaces.

basin in South American has increased rates of terrestrial water The method works, because the radar pulse is backscattered twice

6 S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10

Fig. 6. The trend of terrestrial water storage from GRACE (2002–2011).

(“double-bounce” – Richards et al., 1987) from the water surface variations. For example, Fig. 6 shows rapid melting of glaciers in

and vegetation. Although most vegetation scattering theories sug- Greenland, West Antarctica and Alaska.

gest that the short wavelength radar signal (X- and C-band), as

well as the cross-polarization signal, interacts mainly with upper

4.6. Crustal deformation monitoring

levels of the vegetation and, hence, should be suitable for repeat

pass interferometry, the analysis of all data types with different

Space geodetic techniques, particularly the denser GPS obser-

polarizations have indicated the suitability of all inSAR data to the

vations, provide new tools to monitor and model the present-day

wetland application (Hong et al., 2010a; Hong and Wdowinski,

crustal deformation associated with hazards. For example, East Asia

2011). However the data quality, as calculated from interferometric

is located in a complex convergent zone with several plates, e.g.,

coherence, strongly depends on the time span between the InSAR

Paciﬁc, North American, Eurasian, and Philippine Sea plates (Jin

acquisitions. Short wavelength signals (X- and C-band) maintain

and Zhu, 2003). Subduction of the Philippine Sea and Paciﬁc plates

coherence when the acquisition time span is in the range of 1–70

and expulsion of Eurasian plate with Indian plate collision mak-

days, depending on the acquisition parameters and the vegetation

ing this region one of the most seismically active and deforming

type, whereas the longer wavelength L-band can maintain coher-

regions in the world. Current deformation in East Asia is distributed

ence up to three years in a woody wetland environment (Kim et al.,

over a broad area extending from Tibet in the south to the Baikal

2013).

Rift zone in the north and the Kuril-Japan trench in the east. The

Wetland InSAR works very well in wetlands and ﬂoodplains

interplate deformation and interaction between the blocks are very

enabling high spatial resolution detection of water level changes

complicated and active. Because of low seismicity and uncertainties

with 5–10 cm level accuracy (Wdowinski et al., 2004, 2008). By

in the geographical boundary of faults, it is difﬁcult to accurately

combining the InSAR observations with ground-measured stage

determine the crustal deformation and describe the tectonic fea-

(water level) data, it is possible to generate high spatial resolution

tures and evolution of the deformation belts in East Asia. With

maps of absolute water levels and their changes over time (Hong

more GPS observations in East Asia, the system will provide new

et al., 2010b). Wetland InSAR observations are useful for wetland

ways to clearly monitor the detailed crustal deformation, e.g., the

monitoring by water authorities, characterization of tidal ﬂush-

national projects “Crustal Movement Observation Network of China

ing zone (Wdowinski et al., 2013), and constraints of high spatial

(CMONC) with more than 1000 GPS sites and the Japanese GPS

resolution surface ﬂow model.

Earth Observation Network (GEONET) with more than 1000 con-

tinuous GPS sites. These dense GPS observations will obtain a more

accurate estimate of crustal deformation in East Asia. Fig. 7 shows

4.5. Cryosphere mapping

crustal deformation rates in the Eurasia plate (EU) ﬁxed reference

frame with error ellipses in 95% conﬁdence limits. The possibility of

Glaciers can also be monitored by radar remote-sensing (e.g.,

micro-plate motion independent of the Eurasian plate was tested

InSAR). However, coverage is limited and costs are high. As the ice

and the characteristics of tectonic activities were given using GPS

and snow thickness are related to the amplitude of the reﬂected

derived velocities (Jin et al., 2007).

signal as a function of the incidence angle or relative amplitudes

between different polarizations, the snow and ice thickness can

be retrieved from the GPS reﬂected signals. Komjathy et al. (2000) 4.7. Geophysical loading

derived the condition of sea and fresh-water ice as well as the

freeze/thaw state of frozen ground from aircraft experiments with Continuous GPS observations have signiﬁcant seasonal changes,

GPS reﬂections over the Arctic sea ice and ice pack near Barrow, which are mostly excited by the redistribution of the surface ﬂuid

Alaska, USA. In addition, the change in snow depth is also related mass (including atmosphere, ocean, and continental water) (van

to the corresponding multi-path modulation of the ground-based Dam and Wahr, 1987; van Dam et al., 2001; Dong et al., 2002). How-

GPS signal, which can be monitored by GPS (Jacobson, 2010). In ever, various empirical models have larger differences, particularly

addition, GRACE can determine well the glacier and ice-sheet mass hydrological models due mainly to the lack of a comprehensive

S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10 7

BRAZ 25 GPS 20 GRACE MODEL 15

-5

-10

Vertical displacement residuals(mm) displacement Vertical -15

-20

200 2 2003 2004 2005 2006 2007 2008 2009

Time(y ear)

Fig. 7. Crustal deformation rates in the Eurasia plate (EU) ﬁxed reference frame with

error ellipses in 95% conﬁdence limits.

Fig. 9. Monthly vertical displacement time series from GPS, GRACE and geophysical

models at BRAZ site.

Jin et al., 2005). It needs to be further investigated using longer and

global network for routine monitoring of the appropriate hydrolog-

more GPS and GRACE measurements.

ical parameters. The satellite mission, GRACE, has provided global

surface ﬂuid mass transport for more than a decade. The vertical

loading displacements from GPS, GRACE and geophysical models 4.8. Geocenter and Earth’s oblateness variations

are computed and compared, and at most global sites, the root mean

square (RMS) of GPS coordinate time series is reduced after remov- The transfer and redistribution of the Earth’s atmosphere,

ing the GRACE and models estimates, and the annual variations of ocean and land water masses due to the tectonic activities and

the GPS height at most sites agree well with GRACE estimates in global climate change can lead to variations of Earth’s oblate-

the amplitude and phase (Fig. 8) (Zhang et al., 2012). For example, ness (J2) and geocenter motion (Jin et al., 2010a, 2011b). Satellite

Fig. 9 shows the vertical displacement time series from GPS, GRACE laser ranging (SLR) can well estimate the geocenter motion and

and geophysical models at BRAZ site. It indicates that the nonlinear degree-2 zonal gravitational coefﬁcient variations, but subjecting

seasonal GPS vertical displacement variations are mainly caused to sparse and unevenly distributed SLR stations as well as higher-

by the geophysical loading effects. However, at some sites, partic- altitude laser satellites. Although the new generation of satellite

ularly in the Antarctica, some ocean coasts, small peninsulas and gravity GRACE has largely improved the lower-order coefﬁcient

Europe, discrepancy has been found between the GPS and GRACE estimates with orders of magnitude, but is not sensitive to geo-

estimates (e.g., van Dam et al., 2007). The remaining disagreement center and J2. Nowadays, continuous GPS observations can provide

may be due to the GPS technical errors and GRACE accuracy (e.g., high precision 3-dimensional coordinate time series, which can

80oN

40oN e

ud 0o

latit GRACE 10 mm

40oS GPS 10 mm MODEL 10 mm

o 1 1 80 S 8 W 2 0 o W o 6 0 o 0 o 0 0 o W o W o E 8 2 0 0 E 1 1 6

longitude

Fig. 8. Annual variations of vertical displacements from GPS, GRACE and geophysical models.

8 S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10

(a) Px GPS GPS+OBP GPS+OBP+GRACE SLR GRACE Models 8 40 Geod.-Wind-Current Models GRACE 6 20 4 0

) 2 -10 -20 0 (X10 2 J Δ -2 Polar motion excitation (mas) -40 2003 2004 2005 2006 2007 2008 2009 2010 -4 Time (year) (b) Py -6 60 Geod.-Wind-Current Models GRACE

-8

2003 2004 2005 2006 2007 2008 2009 40 Time (year) 20

Fig. 10. The time series of Earth’s oblateness variations.

-20

estimate low degree gravity ﬁeld coefﬁcients (Wu et al., 2006; -40

Polar motion excitation (mas)

Jin and Zhang, 2012). Here, the GPS coordinates time series from -60

2003 2004 2005 2006 2007 2008 2009 2010

ITRF2008 solutions (Altamimi et al., 2011) are used to estimate

Time (year)

geocenter and J2 together with OBP (ocean bottom pressure)

and GRACE, respectively, which show good agreement with SLR

Fig. 11. Surface ﬂuid mass excitations of polar motion from geodetic

solutions and geophysical models estimates in seasonal, intrasea- observation residual (Geod.-Wind-Current) (blue line), geophysical models

sonal and interannual variations. For example, Fig. 10 shows the (NCEP + ECCO + GLDAS) (green line) and GRACE (red line). (For interpretation of the

references to color in this ﬁgure legend, the reader is referred to the web version of

time series of Earth’s oblateness variations from GPS, GPS + OBP,

the article.)

GPS + OBP + GRACE, SLR, GRACE and Models (Jin and Zhang, 2012).

The variations of J2 at seasonal, intraseasonal and interannual scales

are mainly driven by the atmosphere, oceans and hydrology. ionospheric or electromagnetic observations, can help in this task. A

case study of the 2008 Wenchuan earthquake was performed using

4.9. Geophysical excitations of Earth rotation continuous GPS measurements in China. Signiﬁcant ionospheric

disturbances are found at continuous GPS sites near the epicenter

The Earth’s rotation variables are changing from seconds to with an intensive N-shape shock-acoustic wave propagating south-

decades of years, including polar motion (X and Y) and change eastward, almost consisting with seismometer, indicating that the

of rotation rate (or length-of-day, LOD). At timescales of a few co-seismic ionospheric TEC disturbances were mainly derived from

years or less, the Earth’s rotational changes are mainly driven by the main shock. For example, Fig. 12 shows the signiﬁcant TEC vari-

mass redistribution in the atmosphere, oceans and hydrosphere ations during the mainshock, indicating a signiﬁcant ionospheric

from geophysical ﬂuid models (e.g., Barnes et al., 1983; Gross et al., disturbance (Afraimovich et al., 2010; Jin et al., 2010b) that occurs

2003). However, accurately assessing the atmospheric, hydrologi- simultaneously with the co-seismic rupture. Furthermore, the co-

cal and oceanic effects on polar motion and length-of-day variations seismic tropospheric anomalies during the mainshock are also

remains unclear due mainly to the lack of global observations (Jin found, mainly in the zenith hydrostatic delay component (ZHD) (Jin

et al., 2012). Here the atmospheric, hydrological and oceanic mass et al., 2011c). This conclusion is supported by the same pattern of

excitations to polar motion are investigated from geophysical ﬂuid surface observed atmospheric pressure changes at co-located GNSS

models (NCEP + ECCO + GLDAS) and Gravity Recovery and Climate site that are driven by the ground-coupled air waves from ground

Experiment (GRACE) for August 2002 until August 2009. Results vertical motion of seismic waves propagation. Therefore, the co-

show that the GRACE explains the geodetic residual polar motion seismic atmospheric disturbances appear to indicate an acoustic

excitations at annual periods better (see Fig. 11). However, the coupling between the atmosphere and solid-Earth with air wave

geophysical ﬂuid models do a better job of capturing the intrasea- propagation from the ground to the top atmosphere.

sonal geodetic residuals of polar motion excitation in the Px and Py

components. In addition, GRACE and the combined GRACE and SLR 5. Challenges and future developments

solutions better explain the geodetic LOD excitations at annual and

semi-annual time scales. For less than 1-year time scales, GRACE- Although high precision space geodesy plays a key role in

derived mass does worse at explaining the geodetic residuals, while observing and understanding the Earth system, a number of

SLR agrees better with the geodetic residuals. However, the com- questions still remain, e.g., the precision and temporal–spatial

bined GRACE and SLR results are much improved in explaining resolution of the techniques. For example, currently VLBI cannot

the geodetic residual excitations at intraseasonal scale (Jin et al., continuously observe and SLR has a few global tracking stations,

2011b). particularly in the southern hemisphere. Furthermore, laser ran-

ging satellites are not sensitive to the high-frequency variations

4.10. Atmospheric-solid Earth coupling of low-degree gravity ﬁeld because of the high altitude of their

orbits, e.g., LEGEOS 1/2 with altitude of 6000 km (Cheng and Tapley,

GNSS/InSAR and strong motion seismic measurements provide 2004). Satellite gravimetry, in particular GRACE, is also restricted

unique insights into the kinematic rupture and the size of the earth- in precision and resolution due to its orbital altitude, orbital incli-

quake (Jin et al., 2010b). However, based on current measurements nation, hardware noise and ﬁltering methods (Swenson and Wahr,

understanding and predicting earthquakes is still challenging and 2006). In addition, the accuracy of geophysical models, post-glacial

difﬁcult. Possibly non-seismic measurements, e.g., atmospheric, rebound (Zhang and Jin, 2013) and tide models also affect mass

S. Jin et al. / Journal of Geodynamics 72 (2013) 1–10 9

roles in monitoring and understanding the processes and inter-

actions between the components of the Earth system with a

high resolution and accuracy. Recent results of observing and

understanding the Earth system variations are presented, e.g.,

atmospheric–ionospheric variations, sea level change, hydrologic

cycle, glacier melting, crustal deformation and loadings, geocen-

ter motion and J2 variations, excitation of Earth rotation and

atmospheric-solid Earth coupling etc. Furthermore, some questions

and challenges on observing and understanding the Earth system

variations have been discussed, as well as future developments in

space geodesy.

Acknowledgement

This research is supported by the Main Direction Project

of Chinese Academy of Sciences (Grant No. KJCX2-EW-T03),

Shanghai Science and Technology Commission Project (Grant No.

12DZ2273300), Shanghai Pujiang Talent Program Project (Grant No.

11PJ1411500) and National Natural Science Foundation of China

(NSFC) Project (Grant Nos. 11173050 and 11373059).

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